1. Field of the Invention
The present invention relates to an ultrasonic transmitter employing an electrostatic ultrasonic transducer for remotely controlling radio and television receivers.
2. Description of the Prior Art
Ultrasonic remote-control systems for radio and television receivers are often operated by a number of different command or control signal frequencies, such as eight, which are distributed over the available frequency range of the ultrasonic transmission path. At the transmitting end there is used a simple as possible battery-operated and, therefore, current-saving oscillator which is switched on either by mechanical contacts or electronic sensor circuits as described in U.S. Pat. No. 3,869,671 issued Mar. 4, 1975, and tuned to the selected command or control signal frequency. For this relatively low power type of ultrasonic radiation, besides the piezo-electric arrangements, there are chiefly used electrostatic transducers offering the following possibilities:
1. The a.c. voltage is applied directly to the electrodes of the transducer, while double the frequency of the a.c. voltage is transmitted as sound. PA1 2. The a.c. voltage is superimposed upon a d.c. voltage, for polarizing the electric field between the electrodes of the transducer. The radiated sound frequency is in agreement with the a.c. voltage frequency. This mode of operation has a higher efficiency. In more simple types of well-known electrostatic sound transducers, the dielectric between the electrodes consists of a plastics foil. Operation is done with effective a.c. voltages of about 100 V, and with an additional polarization voltage of at least the peak value of this a.c., in cases where the transducer is operated according to (2) above. For this purpose the supplying a.c. voltage is rectified.
The oscillator, too, is often a simple transistorized oscillator stage of the known type whose radio-frequency coil is provided with a corresponding transformer winding for generating the relatively high a.c. voltage. This arrangement, however, has the disadvantage that in the case of a direct coupling of the electrostatic transducer, the self-capacitance thereof which is often very instable, having an influence upon the frequency stability of the oscillator. Increasing the basic capacitance with a view to reducing this influence, in turn, has the disadvantage of a higher battery current consumption. A decoupling output stage used behind the oscillator and comprising a correspondingly wide-banded output transformer for the working frequency range would have the same disadvantage.